Epidemiologic studies have found that vigorous levels of physical fitness and activity are associated with reduced risk of coronary heart disease (CHD) as well as total mortality. CHD rates remain by far the leading cause of death in the United States. Given that physical fitness and activity are potentially very modifiable risk factors, studies to further define these relationships are extremely important and timely. From 1971-1991 the Occupational Health Service of Los Angeles Country measured physical fitness and other CHD risk factors (including physical activity, anthropometric measures, blood pressure, smoking, family history of CHD, and levels of total cholesterol, uric acid, and glucose) in Los Angeles County firefighters at application and routine intervals (approximately every three years). We now propose to trace this entire cohort of 3403 firefighters for mortality up to 1996. We will use a variety of tracing sources, including the National Death Index, and obtain death certificates to determine date and cause of death. Cohort data will be analyzed to address the following aims: (1) To provide a longitudinal description of and determine the interrelationships among physical fitness, physical activity, and other measured risk factors; (2) To determine whether levels of physical fitness are related, independently of other known and measured risk factors, CHD and all- cause mortality; (3) To determine whether there is a gradient of decreasing CHD risk with increasing levels of physical fitness, or whether there appears to be some threshold of physical fitness, above which CHD risk stabilizes; (5) To determine whether the relationship between physical fitness and CHD and all-cause mortality varies by such cardiovascular risk factors as age, cholesterol levels, blood pressure, smoking, and obesity. Given the comparatively high incidence of death due to cardiovascular disease, and the large number of adults in the United States, who are physically inactive and unfit, addressing these questions in our cohort will have an important impact with regards to public health intervention programs and recommendations.